This invention relates to thin-film electroluminescence apparatus and, more particularly, to a thin-film electroluminescence apparatus suitable for thin-film flat displays for use with information terminal of office automation systems.
A display based on a thin-film electroluminescence (hereinafter referred to simply as "thin-film EL") apparatus has been proposed which has a construction described below. FIG. 1 shows a structure in which a dielectric layers 4 and 6 are provided on two sides of a fluorescent material layer 5, and these layers are interposed between a transparent electrode 2 and a back electrode 7. Thin-film EL displays in which ZnS: Tb, F for green luminescence or ZnS: Mn for orange luminescence is used for the fluorescent material layer 5 are known. In all cases, emitted light is extracted through a glass surface on one side of the layers where the transparent electrode is provided, and the intensity of light thereby extracted is at most about 10% of that of the light emitted from the emission center of the fluorescent material layer.
This cause is based on the Fresnel's law, that is 90% or more of the light emitted from the emission center of the fluorescent material layer is reflected by the interface between the fluorescent material layer and the dielectric layer or between the latter and the transparent electrode. This is because the angle of total reflection to the emission wavelength is considerably small, that is, it is about 25.degree..
On the other hand, a method is known in which a Fabry-Perot interferometer is used for selecting the wavelength of light emitted from a light source having a wide range of emission wavelength. The Fabry-Perot interferometer allows transmission of light only when the light satisfies the following optical interference condition: EQU L.multidot.q=K.multidot..pi.(.pi.: circular constant)
where L represents the distance between a pair of reflecting mirrors 8 disposed parallel to each other as shown in FIGS. 2a and 2b, q represents the number of waves between the reflecting mirrors, and K is a positive integer. It has been actually found that as the reflectivity R of the reflecting mirrors is increased, the half width of the spectrum of light becomes narrower, as shown in FIGS. 3a and 3b. This phenomenon is described on pages 51 to 56 of Laser Physics Nyumon (Introduction to Laser Physics) written by Khoichi Shimota (published on Apr. 22, 1983 by Iwanami Shoten).
It is also known that this interferometer can be used as a laser resonator if a laser medium is inserted in the interferometer.
A thin film interposed between repetition multilayer films (multilayer-film optical interference filter) has a structure such as that shown in FIG. 4. It has been revealed that the interference characteristics of a thin film having this type of structure including reflecting layers formed on two sides of the film and having a high reflectivity ensure the same effects as the Fabry-Perot interferometer, as shown in FIG. 5. This type of thin film is formed by laminating optical thin films having different refractive indexes while setting the film thicknesses so as to satisfy the conditions for prevention of reflection with respect to the emission wavelength .lambda., that is, (n.multidot.d=(1/4+m/2).multidot..lambda. where n represents the refractive index, d represents the film thickness, and m=0, 1, 2 . . . ). Explanations relating to this thin film are found on pages 30 to 34 and 98 to 129 of Optical Thin Film edited by Shiro Fujiwara (published on Feb., 25, 1985 by Kyoritsu Shuppan).
The thin-film EL apparatus shown in FIG. 1 has an advantage in being easily manufactured, and thin-film EL displays based o this apparatus have been put to practical use. However, colors of these displays are limited to orange based on the use of ZnS: Mn for the fluorescent material layer and green based on the use of ZnS: Tb. To manufacture a thin-film EL display capable of displaying three elementary colors, materials for the fluorescent material layer are required which enable emission of light having red and blue emission colors with a high emission efficiency, but fluorescent layer materials have been not yet developed for realization of a practical display. Further it has been very important to improve the emission efficiency.